This invention relates to glow discharge apparatus comprising a body at least part of which is at least partly light-transmissive, the body defining a cavity containing a gas, the glow discharge being powered by one or more drive electrodes external to the cavity, and a secondary light source for illuminating the gas to provide electrons in the cavity, thereby assisting discharge initiation in intermittent use.
Lamps employing such a discharge are often called "electrodeless" lamps, although electrodes for other purposes may be present in the cavity.
A known apparatus of this type disclosed in EP-A-0 607 633, which describes an electrodeless lamp having a fill comprising mercury vapour, and a layer of photoluminescent material. The photoluminescent material is used to convert the UV light produced by the glow discharge to visible light. Such lamps have the disadvantage that operation at low temperatures is not reliable due to the low vapour pressure of mercury. The efficiency of the lamp for visible light is also not as high as that possible if no photoluminescent material were necessary.
According to the invention there is provided a glow discharge apparatus as defined in the first paragraph above, characterised in that the gas predominantly comprises one of the group consisting of neon and xenon. This enables visible light to be directly produced without the problems provided by the presence of mercury and/or photoluminescent material. The use of noble gases in this application is surprising because it is generally acknowledged that such discharges are much more difficult to start in intermittent use than mercury based discharges. The inventors are not aware of any teaching that the provision of a secondary light source is advantageous for electrodeless discharges in any noble gas in the absence of mercury vapour.
The invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, in which